Field of the Invention
The disclosure relates to a sensor, a strain sensor, and a pressure sensor.
Priority is claimed on Japanese Patent Application No. 2014-180175, filed Sep. 4, 2014, the contents of which are incorporated herein by reference.
Description of Related Art
Strain gauge is a typical example of a sensor which measures strain of a measurement specimen. The strain gauge is roughly classified into a metal strain gauge and a semiconductor strain gauge. The metal strain gauge is a strain gauge which has a metal foil which is an isotopic conductor, a thin wire, and so on. The semiconductor strain gauge is a strain gauge which utilizes piezo resistance effect of semiconductor (effect of changing electrical resistivity of semiconductor by stress). Additionally, there is a resonant strain gauge in which a resonator is formed on a semiconductor wafer by a MEMS (Micro Electro Mechanical Systems) technology. The resonant strain gauge has the resonator, a supporter which fixes both ends of the resonator, and a substrate for adding strain to the supporter. Resonant frequency of the resonator is changed by the strain of the supporter. The resonant strain gauge detects the strain by detecting the change of the resonant frequency of the resonator.
Gauge factor of the metal strain gauge is about more than 1 and about 10. On the other hand, gauge factor of the semiconductor strain gauge is about 100, and gauge factor of the resonant strain gauge is about 1000. The semiconductor strain gauge is higher in sensitivity of strain than the metal strain gauge, and the resonant strain gauge is further higher in sensitivity of strain. Therefore, in a case of measuring fine strain, the semiconductor strain gauge is used more often than the metal strain gauge. In a case that strain of measurement specimen is generated by pressure applied to the measurement specimen, the strain gauge is also used so as to measure the pressure applied to the measurement specimen.
For example, the metal strain gauge is used in a state of being attached to the measurement specimen with an organic adhesive such as polyimide. Because a temperature error of the metal strain gauge is caused by a difference between a thermal expansion coefficient of the isotopic conductor, which is used in the metal strain gauge, and a thermal expansion coefficient of the measurement specimen, the metal strain gauge of which thermal expansion coefficient approximates to the thermal expansion coefficient of the measurement specimen is used so as to decrease the temperature error.
In WO 2002/035178, an example of a semiconductor strain gauge is disclosed. In Japanese Patent No. 4511844, an example of measuring pressure applied to a measurement specimen by using a strain gauge is disclosed. In “EXAMPLE OF BONDING METHOD AND MOISTURE-PROOF PROCESSING OF STRAIN GAUGE, [online], [searched on Aug. 12, 2014], Internet<http://www.kyowa-ei.com/jpn/technical/notes/bonding_procedure/index.html>, a specific adhering method for adhering a strain gauge to a measurement specimen is disclosed.
As described above, in a case that the metal strain gauge is attached to the measurement specimen with the organic adhesive such as polyimide, a stress is applied to the organic adhesive by a difference between a thermal expansion coefficient of the metal strain gauge and a thermal expansion coefficient of the measurement specimen. Therefore, if temperature is repeatedly changed and stress is repeatedly applied to the organic adhesive, there is a problem that displacement and release of an adhesive interface easily occur, and drift and hysteresis occur. Recently, a long-term use of several decades of a strain gauge is required. However, because the drift and the hysteresis significantly worsen a measurement result of the metal strain gauge which is used in a long-term, there is a need to form the structure without using any adhesive.
On the other hand, so as to measure with high accuracy, there is a problem as follows. A case that the measurement specimen is made of steel or concrete will be described in detail. So as to measure a fine strain generated in steel and concrete in high accuracy, it is thought that the semiconductor strain gauge which is higher in strain sensitivity than the metal strain gauge is used. However, because a difference between a thermal expansion coefficient of the semiconductor strain gauge and a thermal expansion coefficient of steel and concrete is large, a temperature error is large. Therefore, there is a problem that a fine strain generated in steel and concrete cannot be measured in high accuracy.